What Is a Wire Gauge Calculator and How Does It Work?
A wire gauge calculator determines the minimum conductor size needed for a circuit by checking two limits at once: the wire's current-carrying capacity (ampacity) and the voltage lost to resistance over the run (voltage drop). It converts your current, supply voltage, wire length, and conductor material into a recommended American Wire Gauge (AWG) size.
The voltage drop formula used by electricians is: VD = (2 × K × I × L) / CM for single-phase or DC circuits, and VD = (1.732 × K × I × L) / CM for three-phase circuits. Here, K is the resistivity constant (12.9 for copper, 21.2 for aluminum at ~75°C), I is current in amps, L is the one-way length in feet, and CM is the conductor's cross-sectional area in circular mils.
According to the National Electrical Code (NEC), branch-circuit conductors should be sized so voltage drop does not exceed 3% for the branch circuit alone, or 5% for the combined feeder and branch circuit, to keep equipment operating efficiently and avoid overheating.
How to Use This Wire Gauge Calculator
Enter your circuit values and the recommended AWG size updates instantly. Here's what each field means:
- Current (Amps): The maximum current the circuit will draw — check the breaker rating or the connected appliance's nameplate.
- Supply Voltage: The circuit's nominal voltage, e.g. 120V or 240V for US residential circuits, 12V/24V for DC/automotive, or 230V for many international systems.
- Wire Run Length: The one-way distance from the power source to the load, in feet or meters. The calculator automatically doubles this for the round-trip resistance in single-phase circuits.
- Conductor Material: Copper has roughly 40% lower resistance than aluminum of the same size, so aluminum runs typically need to be sized up.
- Max Voltage Drop: The percentage of supply voltage you're willing to lose to resistance — 3% is the NEC-recommended default for branch circuits.
This calculator works with both US customary AWG sizing and metric wire lengths, and supports single-phase, DC, and three-phase circuits — useful whether you're wiring a US garage sub-panel or sizing a European three-phase motor feeder.
AWG Wire Size Reference Chart
American Wire Gauge (AWG) numbers run in reverse of what you might expect — a lower number means a thicker wire. The table below shows circular mils and 75°C ampacity ratings for common sizes, based on NEC Table 310.16.
| AWG Size | Circular Mils | Copper Ampacity | Aluminum Ampacity |
|---|---|---|---|
| 14 AWG | 4,110 | 20 A | Not rated |
| 12 AWG | 6,530 | 25 A | 20 A |
| 10 AWG | 10,380 | 35 A | 30 A |
| 8 AWG | 16,510 | 50 A | 40 A |
| 6 AWG | 26,240 | 65 A | 50 A |
| 4 AWG | 41,740 | 85 A | 65 A |
| 2 AWG | 66,360 | 115 A | 90 A |
Source: NEC Table 310.16, 75°C column — general-purpose branch-circuit ratings for reference only. Ampacity can be derated for ambient temperature, conduit fill, and conductor bundling.
Frequently Asked Questions
What size wire do I need for a 20 amp circuit?
A 20-amp, 120V circuit typically needs 12 AWG copper wire, which is rated for 25A at 75°C — comfortably above the 20A load per NEC guidelines. For runs longer than about 75 feet, upsize to 10 AWG to keep voltage drop under 3%.
How accurate is this wire gauge calculator?
This calculator applies the standard NEC voltage-drop formula and 75°C ampacity table used by most online wire-size tools, giving a reliable estimate for general planning. It does not account for ambient temperature above 30°C, conduit fill with multiple current-carrying conductors, or insulation type (60°C/90°C), all of which can further derate a wire's safe ampacity. For code-compliant installations, verify sizing with a licensed electrician.
What is the difference between wire gauge and circular mils?
Wire gauge (AWG) is a standardized numbering system for wire diameter — lower numbers mean thicker wire. Circular mils (CM) measure the actual cross-sectional area of the conductor, calculated as the diameter in mils (thousandths of an inch) squared. Circular mils are used in the voltage-drop formula because resistance is inversely proportional to cross-sectional area, not to the AWG number itself.
How do I reduce voltage drop on a long wire run?
Use a thicker (lower AWG number) wire, shorten the run if possible, or split the load across a higher supply voltage circuit. Doubling the cross-sectional area of the conductor roughly halves the voltage drop for the same current and length.